1. Field
The disclosed embodiments broadly relate to fabrication of composite laminates, and deal more particularly with a method and apparatus for forming thick thermoplastic composite structures.
2. Background
Fiber reinforced thermoplastic laminates may be fabricated by assembling a stack of pre-preg plies, and consolidating the ply stack into a finished part. Consolidation is achieved by heating the plies to their melt temperature and molding the ply stack to the desired part shape using conventional compression molding, continuous compression molding or other techniques. During molding, slippage of the plies relative to each other allows the ply stack to change shape and conform to the geometry of a mold tool. Thin thermoplastic laminates comprising relatively few pre-preg plies may be fabricated without difficulty using continuous compression molding, in part because the heat required to melt the thermoplastic travels relatively quickly throughout the thickness of the laminate.
Problems may arise, however when fabricating thermoplastic composite laminates that are relatively thick, especially those having complex geometries. When the thermoplastic resin melts during consolidation and forming, excessive material movement required for consolidation allows reinforcing fibers to move and distort both in-plane and out-of-plane.
Another problem in forming thick thermoplastic composite laminates is caused by excessive material bulk resulting from the need for a large number of plies in the ply stack. Ply stacks that are particularly thick may be difficult or impossible to consolidate and mold to shape. Material bulk may be partially accounted for by making adjustments in mold tooling, however it may nevertheless be difficult to fully consolidate the ply stack during forming. The inability to fully consolidate thick ply stacks due to excessive material bulk, may lead to porosities and internal voids in the finished part.
Accordingly, there is a need for a method and apparatus for fabricating thick thermoplastic composite laminates which de-bulks and partially consolidates a thermoplastic ply stack prior to molding in order to reduce, wrinkles, porosities and internal voids in the finished part. There is also a need for a method and apparatus as described above which reduces or eliminates fiber distortion as the ply stack is being formed to final shape.